The red flag with a white sun disc and 4 rays was used from 4 January 1897 until 30 January 1914. The colors were reversed (red sun with 4 rays on a white flag) on 30 January 1914 and remained in use until the end of World War II.Nozomi Kariyasu, 23 February 2000

I believe that first time this flag was for transport of troops and later for general maritime transports when a flags for military transports was adopted

Source: Flaggenbuch, 1905

Jaume Ollé 12 October 1999

This flag was officially adopted as "maritime transport" under decree 1 December 1932 by the Japanese navy. Actually, this flag was used since beginning 1900's. Prior to that point this flag was called "convoy flag" which shows in Japanese flag books published in 1876 and 1872. Between 1870-1932, this was a "convoy flag" and since 1932, a "maritime transport flag".Nozomi Kariyasu

The flag was adopted on 29 November 1871 for use by the Navy in convoys. Durin 7 October 1889 and 1 December 1932 it was used for Navy transport. From 1 December 1932 until the end of World War II, it was a naval flag for general use.Nozomi Kariyasu, 23 February 2000